Spatial light modulators (SLMs) are increasingly used in projection display applications. A DMD is a type of SLM having an array of micro-mechanical mirror elements, each individually addressable by electronic data. Depending on the state of its addressing signal, each mirror element is moved so that it either does or does not reflect light to an image plane.
For SLM-based display applications, the incoming video signal must be converted to binary data in a form useable by the SLM. As a result of being converted from analog to digital form, the data is first arranged pixel-by-pixel, row-by-row, and frame-by-frame. If the data was interlaced, the data may also require scan conversion from fields to frames. For example, a DMD displays one bit per mirror element at a time. In other words, the image reflected by the DMD at any one moment represents a set of bits having the same binary weight. Thus, before delivery to the SLM, the data must be reformatted into "bit-planes". For pixels having an n-bit resolution, there are n bit-planes per image frame.
U.S. Pat. No. 5,278,652, entitled "DMD Architecture and Timing for Use in a Pulse-Width-Modulated Display System", describes a type of DMD-based projection display system. It also describes a method of formatting video data for use with such a system, and a method of modulating bit-planes to provide varying intensity.
Color images are generated from bit-planes representing different colors. As an example, video data might have 24 bits of data per pixel. Assuming that the colors are red, green, and blue, of these 24 bits, 8 bits would be red data, 8 bits for green, and 8 bits for blue. One complete video frame would be comprised of 24 bit-plane images.
One technique for providing a color image is to use a white light source and a color filter in front of the SLM. In one such design, the white light illuminates the SLM, and the color filter is placed between the SLM and the image plane, so that the image reflected from the SLM is filtered with that color. In another such design, the color filter is placed between the white light source and the SLM.
A common implementation of the color filter approach is to temporally filter the light with a motor-driven "color wheel" having a red section, a blue section, and a green section. As the wheel rotates, red, blue, or green data is transmitted through the corresponding section. The colors of the final image depend on the bit-plane data for each color. U.S. Pat. No. 5,233,385, entitled "White Light Enhanced Color Field Sequential Projection", describes the use of a color wheel for a DMD-based projection display system.
In color wheel applications, the rotational speed and phase of the color wheel and the timing of the image data being reflected from the SLM must be synchronized. In other words, the color wheel must rotate so that the data is transmitted through the correct color at the correct time.
One difficulty of providing a properly synchronized color wheel is that a change from one video signal to another generally results in a change in color phase. Even if the new data is at the same frequency as the old data, the phase changes. For example, in a television system, the viewer may change channels, such that processed blue data from the new channel is available to the SLM at the time as processed red data from the old channel would have been available. As a result, unless the data and the color wheel are re-synchronized, the blue data would be present in the SLM while the red part of the color wheel is in front of the SLM.
One existing technique for re-synchronizing the data and color wheel is to drive the color wheel with a high-torque motor, which can quickly accelerate or decelerate the color wheel to adjust its phase. However, these high-torque motors are expensive.